1. Field of the Invention
The present invention relates to differentials, and more particularly, to limited slip differentials.
2. Description of the Related Art
Differentials are well known in the prior art and allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.
The completely open differential, i.e. a differential without clutches or springs, is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel, for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a "spin out" of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior methods of limiting slippage between the side gears and the differential casing use a frictional clutch mechanism, either clutch plates or a frusto-conical structure, and a bias mechanism, usually a spring, to apply an initial preload between the frictional clutch mechanism and the differential casing. By using a frictional clutch mechanism with an initial preload, for example a spring, a minimum amount of torque can always be applied to the wheel having traction, i.e. the wheel located on dry pavement. The initial torque generates gear separating forces which further engage the frictional clutch and develop additional torque. Examples of such limited slip differentials are disclosed in U.S. Pat. Nos. 4,612,825 (Engle), 5,226,861 (Engle) and 5,556,344 (Fox), which are assigned to the assignee of the present invention and expressly incorporated herein by reference.
The initial preload initiates the development of side gear separating forces which provide further braking action between the side gears and the differential casing. In general, gear separating forces are forces induced on any set of meshing gears by the application of torque to the gears and which forces tend to separate the gears. In a differential, the development of torque will create side gear separating forces which tend to move the side gears away from the pinion gears. On a surface having a low coefficient of friction, the initial preload creates contact and friction pressure between the differential casing and the clutch mechanism disposed between the side gears and the differential casing to allow the engine to develop an initial torque. This initiation of torque transfer induces gear separating forces on the side gears which tend to separate the side gears to further increase friction between the clutch mechanism and the casing. The increased friction pressure of the clutch allows more torque to be developed, thus further increasing the side gear separating forces and limiting the slippage between the side gears and the differential casing.
However, such preloaded clutches are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Additionally, such clutch mechanisms usually employ spring mechanisms which add to the cost and difficulty of manufacture.
Additionally, such a preloaded clutch mechanism may lock the output shafts together in situations where differential rotation is necessary. For example, if the vehicle is making a turn when the wheels are sufficiently engaged on the road surface and a sufficient amount of torque is developed, the differential will tend to lock up the output shafts due to the action of the side gear separating forces created by the developed torque. This may occur, for example, during tight turns on surfaces with a low coefficient of friction. In such a case, even though differential rotation is required, the torque and side gear separating forces lock up the two output shafts causing either wheel to drag and slide along the road surface. This problem is evident in rear drive vehicles during tight turns as the portion of the vehicle near the dragging wheel may tend to bounce up and down.
Another method of limiting slippage involves the use of a frictional clutch between the side gears and the differential casing based on the difference in rotational speeds between the two output shafts. The frictional clutch may be actuated by various hydraulic pump mechanisms which may be external to the differential case or may be constructed of elements disposed inside the differential casing. However, such mechanisms usually are complicated and also add to the cost and difficulty of manufacture.
Thus, what is needed is a simple, durable and reliable limited slip differential which can effectively provide torque to the wheel with traction.
What is also needed is a limited slip differential which is responsive to both the torque and speed difference to provide the limited slip function only when required, i.e. limited slip when one wheel has lost traction and relatively open when sufficient torque is developed.